This invention relates to a process for forming a filter on a substrate, especially a solid state imager or a liquid crystal display device, and to a solid state imager or liquid crystal display device provided with this filter.
The term "filter" or "filter layer" is used herein to mean the whole layer placed upon a substrate to control the passage of electromagnetic radiation to or from this substrate; this filter may have portions of one or more colors. The term "filter element" is used to refer to a single physically continuous element of the filter of the same color throughout; such a filter element may be a dot or a stripe or have a different physical form. The term "set of filter elements" refers to a plurality of filter elements of the same color physically separated from one another. The term "having color" is used to mean "transmitting at least a portion of electromagnetic radiation of a particular wavelength", and does not necessarily refer to visible radiation; as those skilled in solid state imager technology are well aware, such imagers can be made sensitive to wavelengths well beyond the reach of the human eye and there are important applications, such as night vision equipment and aerospace reconnaissance, which require the formation of multiple set of filter elements which pass only predetermined infra-red or ultra-violet wavelengths, even though such filter elements appear opaque to the human eye. The term "baked" is used with reference to filters and refers to an exposure to an elevated temperature for a period sufficient to cause substantial cross-linking of the photoresist in the filter elements, thereby effecting a substantial increase in the stability of these elements; typically the baking of filter elements involves exposure to temperatures of about 2 to 3 hours at a temperature of 140.degree.-150.degree. C. The term "soft baked" is used with reference to the production of filters and refers to an exposure to an elevated temperature for a brief period sufficient to drive off solvent present in a photoresist layer, but insufficient to cause any significant change in cross-linking or stability of this layer; typically, such soft baking is carried out for 1 to 2 minutes at a temperature of about 90.degree.-100.degree. C.
To obtain a color image using solid state imagers such as charge coupled devices, optical filters in a multicolor stripe or mosaic form are employed; in many cases, these filters are formed directly upon the photosensitive surface of the solid state imager. Similarly, in color liquid crystal display devices, optical filters in a multicolor stripe or mosaic form are provided to control the color of the light which is reflected from, or passes through, the "light gate" provided by each individual liquid crystal pixel. Both these types of filters are normally provided with elements having two or three differing colors. For example, a two color filter may have yellow and cyan elements which overlap in part, with the overlap area providing, in effect, a green element. A three color filter will typically have red, green and blue, or cyan, magenta and yellow elements.
A number of processes are described in the art for preparing such filters. For example, U.S. Pat. No. 4,239,842 describes a process for producing a color filter array by depositing successively on a semi-conductive layer, such as a charge coupled device, a sub-coat, a polymeric mordant, and a photoresist. The photoresist layer is exposed and developed to form a mask, and dye is then heat transferred through the apertures in the photoresist into the polymeric mordant. Finally, the photoresist is stripped.
U.S. Pat. No. 4,565,756 describes a process for forming a color filter. This process comprises laying on a substrate a transparent layer, forming by photolithography in this transparent layer a pattern of filter elements separated by separation regions (grooves or dye-impermeable regions), laying a barrier layer over the transparent layer, forming by photolithography a pattern of apertures in the barrier layer, this pattern of apertures corresponding to the location of a first system of filter elements, dyeing the first system of filter elements through these apertures, and finally removing the barrier layer. The formation of the barrier layer and the subsequent steps of the process are then repeated for other colors.
A variety of techniques have also been developed for producing the fine lines and other image elements needed in the production of integrated electrical circuits and in lithography. For example, U.S. Pat. No. 3,873,361 describes a process for producing thin films for integrated circuits by depositing a) a photoresist (polymeric) layer which is baked to render it non-photosensitive; b) a metallic layer; c) a second photoresist layer, then exposing and developing the second photoresist layer to form a mask, etching the exposed metallic layer through this mask, using the metallic mask so produced to remove exposed polymeric layer, preferably by sputter etching, depositing a metallic film in areas where the bottom polymeric layer has been removed, and finally removing, by conventional lift-off solvent methods, the remaining parts of the metallic layer and the bottom polymeric layer.
U.S. Pat. No. 4,808,501 describes a process for forming a color filter on a support, such as a charge coupled device, by (a) forming a layer on a support with a composition comprising a positive photoresist and a dye, the dye being soluble in the solvent of the photoresist; (b) exposing predetermined portions of the layer to radiation adapted to increase the solubility of the coating in the exposed areas; (c) developing the exposed areas to form a pattern of filter elements; and (d) repeating these steps with a different color dye in the composition; wherein the dye constitutes in excess of 10% by weight, dry basis of the composition, is substantially non-absorptive in the exposure wavelength of the composition, and provides predetermined absorptive characteristics for the specified filter element and the dye possesses substantially the same polarity as the composition. In practice, after the exposed areas have been developed, the patterned photoresist must be baked, typically at 140.degree.-150.degree. C. for 1 to 3 hours, to stabilize the filter elements.
U.S. Pat. No. 5,059,500 describes a process for forming a filter using differential reactive ion etching techniques. This process comprises:
providing on the substrate a layer of an absorber material having predetermined absorption and transmission characteristics; PA1 providing a layer of a barrier material superposed on the layer of absorber material, PA1 the barrier material being more susceptible to reactive ion etching than the absorber material under a first set of etching conditions, but resistant to reactive ion etching under a second set of etching conditions under which the absorber material is susceptible to etching; PA1 providing a layer of a photoresist material superposed on the layer of barrier material; PA1 patternwise exposing the layer of photoresist material and developing the exposed layer to remove either the exposed or non exposed regions thereof, thereby to bare the regions of the barrier layer underlying the removed regions of the photoresist material, the remaining regions of the photoresist material being resistant to reactive ion etching under said first set of etching conditions but susceptible to reactive ion etching under said second set of etching conditions; PA1 reactive ion etching the coated substrate under said first set of etching conditions, thereby etching away the bared regions of the barrier layer and baring selected regions parts of the absorber layer, but not etching away the remaining regions of the photoresist material nor substantially etching away the bared regions of the absorber layer; and PA1 reactive ion etching the coated substrate under said second set of etching conditions, thereby etching away the remaining regions of the photoresist layer and the bared regions of the absorber layer, and thereby forming a filter on the substrate. PA1 forming, on the solid state imager, a layer of a first photoresist composition comprising a photoresist and a first dye; PA1 exposing portions of the layer of first photoresist composition to radiation effective to change the solubility of the exposed portions of the photoresist composition; PA1 developing the exposed layer of first photoresist composition by removing from the substrate one of the exposed and unexposed portions of the layer of first photoresist composition, PA1 thereby producing first filter elements from the remaining portions of the layer of first photoresist composition; PA1 thereafter, treating the substrate with a silylation compound having at least two functional groups, this silylation compound being capable of cross-linking the photoresist in the first filter elements, and of promoting adhesion of this photoresist to the substrate; PA1 thereafter forming, on the substrate bearing the first filter elements, a layer of a second photoresist composition comprising a photoresist and a second dye, the second dye having radiation absorption characteristics different from those of the first dye; PA1 exposing portions of the layer of second photoresist composition to radiation effective to change the solubility of the exposed portions of the photoresist composition; PA1 developing the exposed layer of second photoresist composition by removing from the substrate one of the exposed and unexposed portions of the layer of second photoresist composition, PA1 thereby producing second filter elements from the remaining portions of the layer of second photoresist composition; PA1 thereafter, treating the substrate with a silylation compound having at least two functional groups, this silylation compound being capable of cross-linking the photoresist in the second filter elements, and of promoting adhesion of this photoresist to the substrate; PA1 thereafter forming, on the substrate bearing the first and second filter elements, a layer of a third photoresist composition comprising a photoresist and a third dye, the third dye having radiation absorption characteristics different from those of the first and second dyes; PA1 exposing portions of the layer of third photoresist composition to radiation effective to change the solubility of the exposed portions of the photoresist composition; PA1 developing the exposed layer of third photoresist composition by removing from the substrate one of the exposed and unexposed portions of the layer of third photoresist composition, PA1 thereby producing third filter elements from the remaining portions of the layer of third photoresist composition; and PA1 thereafter, treating the substrate with a silylation compound having at least two functional groups, this silylation compound being capable of cross-linking the photoresist in the third filter elements.
The processes described in the aforementioned U.S. Pat. Nos. 4,808,501 and 5,059,500 give excellent results. However, the process of U.S. Pat. No. 5,059,500 requires the use of reactive ion etching equipment. The process of U.S. Pat. No. 4,808,501 requires three separate baking steps and imposes a number of stringent requirements upon the dye. As discussed in this patent, the dye must be sufficiently soluble in the photoresist resin that the relatively concentrated dye solution required for the process can be achieved, without the dye tending to precipitate out, either during the formation of the filter, or during the long service life (of the order of several years) of the solid state imager. Furthermore, the dye must be sufficiently stable in solution and sufficiently stable in the filter elements to withstand, without unacceptable color loss, the thermal or radiation-exposure treatments which are normally required to stabilize the filter elements of each color before the next color is applied; if this thermal or radiation-exposure treatment is omitted, the solvent used to deposit the second layer of photoresist tends to redissolve the first set of filter elements, thereby deforming the filter elements, reducing the selectivity of the filter, promoting cross-talk between the various color channels of light passing through the filter and tending to reduce the resolution of the filter. The dye must also not interfere with development of the exposed areas of the photoresist. Finally, the dye must be substantially non-absorptive in the exposure wavelength of the composition. This combination of requirements greatly limits the choice of dyes which can be used in the process of U.S. Pat. No. 4,808,501, and thus tends to increase the cost of the process. In particular, the requirement that the dye withstand a thermal stabilization treatment eliminates numerous dyes from being used in the process, and increases the concentrations of other dyes which must be used, since many dyes which can survive a thermal stabilization undergo significant color loss during this step. A thermal stabilization treatment also creates other problems, especially reflowing and yellowing of the photoresist, which distorts and discolors the filter elements, thus reducing the resolution and sensitivity of the device, and may reduce yields by rendering certain filters unacceptable.
It has now been found that the number of dyes useful in the process of U.S. Pat. No. 4,808,501 and similar processes for forming filters can be increased by treating the filter elements formed by development of the photoresist with a particular type of silylating compound. Pre-treatment of the substrate with this particular type of silylating compound prior to application of photoresist to the substrate can also help to promote adhesion of the photoresist to the substrate.